How to Measure Bolts For Your Bike - A Simple Guide
If you're thinking of upgrading your bolts, getting the right size bolt is essential. In this guide, we’ll walk you through how to measure bolts correctly, including regular and countersunk bolts.
1. Measuring Bolt Length
a. Regular (Button Head) Bolts
For standard bolts, measure from under the head to the tip of the threads.
Image credit: https://jdcustomsusa.com
📏 Example: A bolt that measures 25mm from just under the head to the tip is a 25mm bolt.
b. Countersunk (Flat Head) Bolts
With countersunk bolts, you measure the full length from the top of the head to the tip.
📏 Example: A countersunk bolt that measures 20mm from top to tip is a 20mm bolt.
🔧 Why? Because countersunk heads sit flush with the surface, so the full length matters for fit.
2. Measuring Bolt Diameter
Bolt diameter is the width of the threaded part, not the head.
- Use a caliper to measure across the threads.
- Don’t measure the bolt head — that varies by style.
🧩 Common bike bolt diameters:
- M5 (5mm) – common for water bottle cages, and small fittings
- M6 (6mm) – common for stems, seatpost clamps
- M8 (8mm) – used for heavy-duty applications like cranks
✅ Tip: "M" stands for metric, and the number is the thread diameter in millimeters.
3. Thread Pitch
Most bike bolts use standard thread pitches, but some may vary.
Image Credit: https://www.omnicalculator.com/construction/thread-pitch
- Use a vernier caliper or ruler to measure the length of the thread, and count the number of threads in that length. The thread pitch is the length divided by number of threads.
- Or, use a thread pitch gauge to match the threads
🔍 Example: M6 x 1.0 x 35mm means a 6mm diameter bolt, 1.0mm thread pitch, 35mm length.
4. Links
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[By Tim, Last Updated: 14 Jul 2025]